Production of caustic soda



Patented July 5, 1949 UNITED STATESy 'ATENT OFFICE 2474316 PRoDUcT'IoN oF cAUs'rIc soDA Hartmut W. Richter, Rahway, N. J., assignjoiiflto Metal & Thermit Corporation, New York,- N. X., a corporation of New Jersey Application october 28, 1947, serial Nu. iii-2,482

8 Claims.

This invention relates to a process for manufacturing caustic soda from sodium carbonate and broadly comprises converting the carbonate by means of tin oxide to sodium stannate and then converting the stannate to caustic soda and tin oxide. The' tin oxide initially employed is continuously regenerated and reused so that the net result of the process is the conversion of sodium carbonate to caustic soda.

More specifically, the process comprises reacting together sodium carbonate and tin oxide at elevated temperatures to form sodium stannate in dry g'ranular form, dissolving the stannate in water. heating the dissolved stannate at a temperature in the range of about 150 C. to the critical temperature of water and at a pressure suificient to maintain a liquid phase throughout said temperature range, whereby tin oxide and free caustic soda are formed, and then separating the tin oxide from the caustic soda.

As will become apparent hereinafter, the invention also provides a method of producing sodium stannate in dry granular form by means of a dry reaction between sodium carbonate and tin oXide.

The invention may be better understood from the description that follows taken in connection with the accompanying drawing in which certain desirable Optional steps are shown by means of dotted lines.

Tin oxide and sodium carbonate from any source, at least a portion of which may be ob'- tained as her'einafter described, are lr'eacted together in a reaction zone I to produce sodium stannate. The reaction may be set forth in the form of an equation as follows:

NazCOs SnOz :NazSnOs CO2 The sodium carbonate may be of any quality,

either pure or of a Commercial grade or in crude form. Preferably the concentrations of the carbonate and tin oxide are the stoichiometricali amounts required by the foregoing equati'on, as it is then possible to obtain a product which lis substantially completely soluble in water and which comprises essentially sodium stann'ate in a dry granular form. It is, however, possible to employ exces's tin oxide without impairing the dry granular form of the stannate, the excess merely appearing in the product. In this case, however, the product will not be completely sol-uble in water. Excess sodium carbonate may also be used, and while this permits a lower reaction temperature to be employed, it results in a fused reaction mixture which is -lessconveniently handled thanl the dry 'granular stannate material. The reaction temperature may be about 8G0-1-200 C., preferably 1000-1l00 Whilethe pressure may be atmospheric. The reaction time is variable, agenerally suitable reaction time being about 30 minutes; although lower and higher times ar'e possiblegsay 1A; to 1 or 2 hours. In generalithe temperature varies with the time, lower temperatures requiring longer times and 'vice versa. Any suitable equipment may be employed for' the reaction, such as, for example, a direct firfe'd 'rotary calciner.

The yield of sodium stannate is quantitative whenv stoichiome'trical amounts 'of the reactants are used. In such case, as stated, the stannate is in dry granular form and easy to handle. If ei;- cess tin oxide is used, the yield of stannate will be quantitative as lresp'e'cts the conversion of the sodium carbon'ate. If excess sodium carbonate is employed, the'yield will be quantitative as regards the conversion of tin oxide.

Following the reaction, the product may be passed through line V2 to dissolve'r 3, Water being added through line llto disselve the material. Preferably, however, 'the 'product is passed directly to an autoclave 5 where it may be heated in the preserfcefof'water'fto diss'olve it preparatory to convertin-g fthe stan-iiat to free -c'aus'tic soda and tin oiiide. Water vifhay'be addedto the autoclavev'ia line 6. It'w'ill be-iindrlstood that other equipment besi-desflan-atoclave may be employed so long as it is suitable for' heat/ing the stanf-fate solution under pressure. For example, a c'ol, which is suitable for batch or continuous operation, may be used. An autoclave, however, has been found to be satisfactory.

In the conversion or autoclave zone 54 the sodium stannalte in aqueo-us solution is converted under heat and pressure ``conditions 'to caustic soda and tin `oxi'de'. The reaction may be expressed by the following equation':

Na2SnOfs+H2`0:2NaOI-I+Sn02 The sta'nnate solution riilaLy'4 V'ary widely in concentration from dilute `s"`olu`tions oontain'in'g, say, l5% by weight'or less ofth'estannate, 'to' slurries, which contain undissolved stannate. Some free alkali vmay ormay net be initial-ly present ri-n'the solution, d'ependir'rg upon the source' of thel initial ca'rbonatematerialand uponpro'ces's variations heremafter' described. Also, less concentrated solutions' than those mentioned may be treated, this being a matter of choice oro'f eco'nomics. The solutions may or' may not contain other' constit'uzis' such as sodium' carbonat'e',

sodium chloride, sodium soaps, and the like, which may be introduced thereto depending upon the quality of the initial carbonate or upon process variations to be described.

of each experiment and leaving a liquid remaining which in the table is referred to as "Filtrate." The tin and free caustic soda concentration of the filtrate are listed, and in each case it will The temperature at which the solution is 5 be noted that the free caustic soda content of heated is desirably in the range of about 150 C. the filtrate is higher than in the original soluto the critical temperature of water (approxition, the gain representing the amount of alkali mately 374 0.). It has been found that the generated from the original stannate, as indiyield of caustic soda tends to be greater at highcated in the last column of the table. Yields er temperatures, and for this reason temperof caustic soda may average about 80% by weight atures in excess of 150 C. are preferred. Prefof the theoretical. The tin content of the filtrate erably, the lower temperature limit is about 180 represents that present in unchanged sodium C. Sufficient pressure is always employed to stannate. maintain a iiquid phase at a11 times during the The precipitated and filtered tin oxide maybe heating. Thus at the initial temperature of 15 passed through line 8 to the reaction zone I to 150 C. the pressure for sodium stannate solutions be reacted with additional quantities of sodium may be about 70 p. s. i. At higher tempercarbonate. The filtrate, which comprises mainly atures the pressure is higher, ranging to 1000- sodum hydroxide and some unchanged sodium 1500 p. s. i. and more. For a given temperature stannate, may be separated in zone 9 into sodium the pressure may be higher than that necessary stannate and sodium hydroxide. The sodium to maintain a liquid phase. It has been found stannate may be recycled via line Ill to join the that for the average stannate solutions comprisstannate material in line 2- The SOdium hying merely stannate and water, the steam-water droxide may be removed as product. Any sultvapor pressure relationship substantially holds. able method Of Sepal'ang the SOdlU-m hydrOXde The time of heating the stannate solution is from the stannate may be employed. For expreferably such as to enable the maximum ample, in one method the filtrate may be evapamount of caustic soda to be formed. Broadly orated until sodium stannate precipitates and speaking, the time may vary from a few minutes this may then be filtel'ed, leaving behind a conto several hours, say 1A; to 3 or 4 hours, but in centrated solution of sodium hydroxide suitable practice, equilibrium between the sodium stanfor Commercial use. In an alternative method, nate and the products resulting from it may the filtrate may be treated with calcium hydroxbe usually attained in about a half hour. In ide to produce calcium stannate as a precipitate. general, the higher the free alkali present in The latter may then be filtered from the solution, the system, the higher will be the temperature leaving the caustic soda solution as product. required, and With higher temperatures the time As shown in the drawing, the filtrate from the W111 tend to be sherten autoclaving step may be passed in an Optional or During the autoclaving reaction tin oxide is alternative step to a detinning operation in zone progressively formed and precipitated, while the ll. Suitable quantities of an oxidizing agent free caustic soda that is formed simply dissolves which is compatible with alkali, say sodium niin the solution. At the end of the reaction 4.) trate, and water, if necessary, may be added to the contents of the autoclave, including small the solution via line |2 to form a detinning solamounts of unchanged sodium stannate, may be vent, comprising, say, 5 to 30% by weight of free passed to a filter zone 'I for separation of the tin caustic soda and 1 to 5% by weight of sodium oxide, and the filtrate may be subjected to a furnitrate. The tin scrap is then treated with this ther separation to remove unchanged sodium solvent in any desired manner. The solvent disstannate. solves all the tin from the scrap to form sodium The autoclaving reaction may be illustrated by stannate and sodium carbonate, the latter being the following table of data in which are prethe result of the carbonation of the sodium hysented the results of a number of runs in each droxide by air or other substances present in the of which a sodium stannate-containing solution Scrap. The detinned metal is removed from the was heated under definite conditions of tin and detinning zone as shown, while the solution may initial free alkali concentrations, temperature, be passed to zone |3 and separated in known pressure and time. manner, say by evaporation, into sodium stan- Material Treated Film-ate Free Caus- Exglme sodium Free Caus- Tf'P' gr -:5. lfi g Free go v e i-i gtd Smmmt, tigclogi, p. s. i. hrs. T/Im NROH, Per Cent Pe'r Cent l g./l.ofT1n NaOH gv g /1 04.5 6.8 235 430 3 29.6 48.7 68.6 94 113.6 10.2 300 1,260 ie 21.3 110.1 84 10o+ 2300 0 300 1,260 1 42.5 132 ss 6t 378.4 9.8 216 1,000 1 15.0 41.2 81

l Based on theoretical amount generable. 2 A slurry.

8An impure'solution, containing sodium carbonate, sodium chlotide, sodium nitrate, sodium sulfate,

soaps, etc.

In each example given the stannate solution appearing in the column Material treated was heated with agitation in an autoclave to the temperature listed and held at such temperature under the time and pressure conditions shown. As a result of such treatment, tin oxide was progressively formed and precipitated in the solunate and sodium carbonate. The stannate may be removed as product and used as desired, while the carbonate, which is recoverable in solid form, is advantageously recycled by line Id to the reaction zone i to react with additional quantities of tin oxide. Alternatively, the solution of stannate and carbonate prior to separation may be tion, being separated therefrom at the conclusion sent to the autoclave v ia line |5 in the event thatV additional Vsodium stannate ymay be required there.

/The following examples may illustratethe invention.

Examp'le `5 As an illustration of the -process shown in the full line flow of the drawing, sodium carbonate and tin oxide are reacted with each other in stoichiometrical concentrations to give a quantitative yield of sodium stannate. Thus, 106 lbs. of carbonate react with 150.7 lbs. of 'tin oxide to give 212.7 lbs. of stannate. This amount Ofstannate is completely dissolved in water and converted to free caustic soda and tin oxide, the conversion being 80% of the theoretical, to form 64 lbs. of caustc soda andv 120.5 lbs. of tin oxide. Twenty per cent of the stannate, or 42.5 ;lbs., remains unchanged, but it becomes hydrated in the aqueous solution to form 53.5 lbs. of the hydrate Na2Sn(OH) e, which may then be separated from the caustic and recycled to the autoclave.

Example 6 lbs. of tin oxide (SnOz) at a temperature of l200 C. The product was then dissolved in water and autoclaved at 3007 C. for about one hour at about 1l00 p. s. i., from which operation there was produced 51 lbs. of tin oxide as a precipitate and a filtrate comprising 10.2 lbs. of tin (in combined form) and 27.6 lbs. of free caustic soda.

It will be evident that out of every 100 lbs. of crude carbonate, which formerly was thrown away, there can be obtained 27.6 lbs. (276%) of free caustic soda of Commercial quality.

In this example most of the tin oxide entering zone i was recovered in the autoclaving step. The deflciency can be made up by recycling the stannate separated from the free caustic soda in zone 9.

It will be apparent that the invention provides a useful method of converting cheap sodium carbonate to the more expensive caustic soda. The advantages of the invention are more marked when it is realized that sodum carbonate from any source may be processed. The cost of carbonate material is proportionate to its quality, so that the cruder the material the greater are the savings that may be realized. As above described, crudes may be treated which heretofore were wasted. In the case of crude carbonate obtained from detinning operations, other valuable materials may also be salvaged, such as tin, without resorting to special recovery steps.

In the light of the foregoing description, the following is claimed:

l. Process of producing caustic soda from sodium carbonate which comprises reacting together stoichiometrical amounts of sodium carbonate and tin oxide at a temperature of about 800 to 1200 C. to form sodium stannate in dry granular form, dissolving said stannate in water, heating the dissolved stannate at a temperature in the range of about 150 C. to the critical temperature of water and at a pressure sufficient to maintain lla .,-li`quldi. phase :throughout said :temperaturev range ;whereby. timoxidesandfree caustic soda are formed,`` some of` said: dissolved'stannate remaining -unchanged, vseparating :the rtin oxide and recycling-it to the ffirst imentioned reaction step to -react =with Ianv additional quantity Avof sodium carbonate,`l adding :sodium nitrate 'to 'the remaining solution of unchanged 'stannate and caustic soda 'to form-a detinning solvent, detinning tin s-crap with-'said solvent to 'form additional Vquantities of lsodium stannate and fialso sodium carbonate, separating -the carbonate' from the stannate, and recyclingithe'carbonate vto the first mentioned reactionfstep to react^with 'an additional quantity oftin oxide.

2. Process of produ-cin'g `v'caustic soda from sodium Vcarbonate which comprises reacting 'together stoichicmetrical :amounts of sodium 'carbonate and tinoxide'at a'temperature offabout 800 to "l`200 Cfito"form-so'dium--stannatein dry granular' form, 'dissolving-said stannate water, heating the 'dissolved :stannate fat' -a temperature in the range of about =l1:50-.C.1to the critical 'temperature of water and at a pressure sufiicient to maintain a liquid phase throughout said temperature range whereby tin oxide and free caustic soda are formed, some of said dissolved stannate remaining unchanged, separating the tin oxide, adding sodium nitrate to the remaining solution of unchanged stannate and caustic soda to form a detinning solvent, detinning tin scrap with said solvent to form additional quantities of sodium stannate and also sodium carbonate. separating the carbonate from the stannate, and recycling the carbonate to the first mentioned reaction step.

3. Process of producing caustic soda from sodium carbonate which comprises reacting together stoichiometrical amounts of sodium carbonate and tin oxide at a temperature of about 800 to 1200 C. to form sodium stannate in dry granular form, dissolving said stannate in water, heating the dissolved stannate at a temperature in the range of about C. to the critical temperature of water and at a pressure sufficient to maintain a liquid phase throughout said temperature range whereby tin oxide and free caustic soda are formed, some of said dissolved stannate remaining unchanged, separating the tin oxide and recycling it to the first mentioned reaction steps to react with an additional quantity of sodium carbonate, separating said stannate from the Icaustic soda and recycling the same to the stannate heating step, and recovering the free caustic soda.

4. Process of producing caustic soda from sodium carbonate which comprises reacting together stoichiometrical amounts of sodium carbonate and tin oxide at a temperature of about 800 to 1200 C. to form sodium stannate in dry granular form, dissolving said stannate in water, heating the dissolved stannate at a temperature in the range of about 150 C. to the critical temperature of water and at a pressure sufficient to maintain a liquid phase throughout said temperature range whereby tin oxide and free caustic soda are formed, some of said dissolved stannate remaining unchanged, separating the tin oxide, separating said stannate from the caustic soda and recycling the same to the stannate heating step, and recovering the free caustic soda.

5. Process of producing caustic soda from sodium carbonate which comprises reacting together stoichiometrical amounts of sodium carbonate and tin oxide at a temperature of about 7 800 to 1200 C. to form sodium stannate in dry granular form, dissolving said stannate in Water, heating the dissolved stannate at a temperature in the range of about 150 C. to the critical temperature of Water and at a pressure sufficient to maintain a liquid phase throughout said temperature range Whereby tin oxide and free caustic soda are formed, separating the tin oxide and recycling it to the first mentioned reaction step to react with an additional quantity of sodium carbonate, and recovering the free caustic soda.

6. Process of producing caustic soda from sodium carbonate which comprises reacting together sodium carbonate and tin oxide at a temperature of about 800 to 1200 C. to form sodium stannate, dissolving said stannate in water, heating the dissolved stannate at a temperature in the range of about 150 C. to the critical temperature of water and at a pressure sufficient to maintain a liquid phase throughout said temperature range whereby tin oxide and free caustic soda are formed, separating the tin oxide, and recovering the free caustic soda.

7. Process of producing caustlc soda froni sodium carbonate which comprises reacting together sodium carbonate and tin oxide at a temperature of about 800 to 1200 C'. to form sodium stannate, dissolving said stannate in water, heating the dissolved stannate at a temperature in the range of about 150 C. to the critical temperature of water and at a pressure sufiicient to maintain a liquid phase throughout said temperature range whereby tin oxide and free caustic soda are formed, and separating the tin oxide.

8. Process of producing sodium stannate in dry granular form which comprises reacting together stoichiometrical amounts of sodium carbonate and tin oxide at a temperature of about 800 to 1200 C. to form a product which is substantially completely soluble in water and which comprises essentially sodium stannate in dry granular form.

HARTMUT W. RICH'I'ER.

No references cited. 

